1. Field of the Invention
The present invention relates to a pentagonal prism module for use in a viewfinder of a single lens reflex camera.
Conventional pentagonal prisms for use in a viewfinder of a single lens reflex camera have all of their three reflection surfaces metal-coated by silver, aluminum or the like. The use of silver for the metal coating provides each of the reflection surfaces with a high reflection factor of 95% but silver is inferior in durability to aluminum and yet higher in cost. Aluminum is lower in cost and more durable but the use thereof for the metal coating provides each of the reflection surfaces with reflection factor of 85% which is lower than that of silver.
There has been proposed, by for example Japanese Patent Laid Open Publication No. Sho 49-64425, a pentagonal prism in which a refractive index of the prism and an angle formed by a roof ridge and a bottom light incident surface of the prism are properly selected so that total reflection of veiwfinder light influx is possible on a pair of roof reflection surfaces of the prism. With this pentagonal prism, only a lower front surface serving as a third reflection surface is metal-coated and therefore, even when aluminum is used for the metal coating, it is advantageous in that the total reflection factor of all of the three reflection surfaces is about 85% which is almost equal to that of the conventional pentagonal prism having all of the three reflection surfaces metal-coated by silver. Yet, it is more durable and manufactured at a lower cost than the conventional pentagonal prisms.
This pentagonal prism making use of total reflection on the roof surfaces, however, poses a problem in camera manufacturing processes. Upon manufacturing of the conventional pentagonal prisms, all surfaces except for a bottom light incident surface and a rear light exit surface are entirely coated with a black light absorbing paint immediately after the completion of the metal coatings on the reflection surfaces and such a paint serves as a protection film of the metal-coated reflection surfaces, thereby necesitating no special attention during camera manufacturing processes. In contrast thereto, with the pentagonal prism making use of total reflection, the roof surfaces as well as the roof ridge should not be subjected to any coating or painting to ensure total reflection thereon, and accordingly it is exposed to dangers, such as dust adhesion and touch of a worker's hand to roof surfaces and destruction of the roof ridge, whereby a careful attention is required during camera manufacturing processes.
From a practical view point, however, even with the pentagonal prism making use of total reflection, all portions of the roof surfaces are not related to forming a viewfinder image field. Thus, peripheral portions of the roof surfaces out of the effective optical path regions are coated with a black light absorbing paint because, if such peripheral portions are left transparent, undesired lights totally reflected thereon produces a ghost in the view field of a viewfinder. Yet, the painting is applied not only to the peripheral portions of the roof surfaces but also to the lower front surface and even to the side surfaces of the prism. In spite of such application of a black paint, the effective optical path regions on the roof surfaces and the roof ridge which are very important for the optical performance of the pentagonal prism remain exposed. Therefore, when the pentagonal prism is assembled into a camera, those exposed portions should be protected from dust adhesion or damages beforehand in order to improve work efficiency.